Previously, when a casting mold (a main mold and a core) used in sand casting is manufactured by way of trial, a molding tool such as a wooden mold, a resin mold or a metallic mold having a reverse structure of a target casting mold has been first designed and manufactured, and then, a trial product of the casting mold has been manufactured using the molding tool. However, it requires much time, professional knowledge, technical skill and the like for designing and manufacturing the wooden mold and the like having such a reverse structure. For this reason, as a new technique used in place of such a conventional process for manufacturing the mold (by way of trial), attention has recently been attracted to a so-called multilayer molding process.
Such a multilayer molding process is a molding process as proposed in patent document 1 (U.S. Pat. No. 5,132,143), and specifically, a technique of directing a laser beam to a sinterable powder scattered in the form of a laminae (first layer) in order to selectively sinter only a necessary portion therein, successively scattering the sinterable powder on the first layer to form a second layer, also directing the laser beam to such a second layer in the same manner as the above in order to selectively sinter only a necessary portion, joining a sintered portion of the second layer and a sintered portion of the first layer sintered by previous beam irradiation, and repeating this process necessary times, thereby multilayering layer by layer to mold a casting mold having a target three-dimensional form.
As the sinterable powder used herein in such a multilayer molding process, there is generally used resin-coated sand similar to that used in shell molding, which comprises refractory particles surfaces which are coated with a resin composition (binder composition). However, such resin-coated sand is required to have properties beyond those of the resin-coated sand used in the shell molding, so that there is employed the resin-coated sand particularly specialized to the multilayer molding process (RCS for a multilayer mold).
As such RCS for a multilayer mold, various ones have conventionally been used. For example, patent document 2 (U.S. Pat. No. 6,335,097) proposes almost-spherical sand particles having a particle diameter of 20 to 100 μm which are coated with resin. It is disclosed that the RCS for a multilayer mold (resin-coated sand for a multilayer mold) is fine particles which have less uneven surfaces and can secure good sand scattering properties, thereby dimensional accuracy of the resulting casting mold can be advantageously secured even when the thickness of a sand layer is as extremely thin as about 0.1 to 0.2 mm.
Further, patent document 2 also discloses that, with respect to the RCS for a multilayer mold (resin-coated sand for a multilayer mold) proposed therein, the resin on the surfaces preferably has a fusion temperature of 100° C. or higher, in order to secure dimensional accuracy of the resulting casting mold, and that the sand particles used therein are preferably mullite-based sand particles, in order to prevent thermal expansion of the sand particles caused by laser beam irradiation and secure dimensional accuracy of the casting mold, and also preventing strain, core cracking and the like caused by thermal deformation at the time when molding is performed using the resulting casting mold. Furthermore, as a specific example in producing the RCS for a multilayer mold (resin-coated sand for a multilayer mold), it is disclosed that a phenolic novolak resin having an average molecular weight of about 2,000 to 10,000 and a fusion temperature of 100° C. or higher is added in an amount of 3 to 5 parts by weight based on 100 parts by weight of sand particles. In addition, patent document 2 also discloses that the multilayer mold manufactured using the RCS for a multilayer mold (resin-coated sand for a multilayer mold) is provided with a vent hole, in order to prevent gas defects caused by pyrolytic products derived from organic substances such as the phenol resin, for example, tar, soot and the like, when molding is performed using the casting mold.
However, in patent document 1 and patent document 2 as described above, only fundamental technical items with respect to the multilayer molding process and the resin-coated sand for a multilayer mold used therein are disclosed. Further, these patent documents point out problems that the resin-coated sand for a multilayer mold is to solve, specifically a problem of sand breaking properties on a boundary face between the multilayer mold which is a solidified layer region and a non-solidified layer region, a problem of gas permeability in the resulting casting mold, and the like, in the casting mold (multilayer mold) in which the strength of the solidified layer (hereinafter referred to as the consolidation strength) is developed by irradiation of a laser beam to such a degree that a subsequent sand scattering operation is performed without trouble, and such solidified layers are sequentially multilayered. However, against such problems, an attempt to improve the RCS for a multilayer mold, specifically, an attempt from the viewpoints of the phenolic novolak resin used in the binder composition which coats a surface of the sand and the sand particle size of the resin-coated sand, is not disclosed at all nor suggested.
Patent Document 1: U.S. Pat. No. 5,132,143
Patent Document 2: U.S. Pat. No. 6,335,097